How are you linking to your css file? '../css/960.css', 'http://rebateninja.com/css/960.css' or '/css/960.css'?
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QoopDec 18 '12 at 2:57

Generally the way to make sure real resources are not rewritten is RewriteCond %{REQUEST_FILENAME} !-f and RewriteCond %{REQUEST_FILENAME} !-d both placed right before the RewriteRule
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Michael BerkowskiDec 18 '12 at 2:59